The PCI devices with DMA masks smaller than 32bit should enable
CONFIG_ZONE_DMA.  Since the recent change of page allocator, page
allocations via dma_alloc_coherent() with the limited DMA mask bits
may fail more frequently, ended up with no available buffers, when
CONFIG_ZONE_DMA isn't enabled.  With CONFIG_ZONE_DMA, the system has
much more chance to obtain such pages.

Bugzilla: https://bugzilla.kernel.org/show_bug.cgi?id=68221
Cc: <stable@vger.kernel.org>
Signed-off-by: Takashi Iwai <tiwai@suse.de>